1. Field of the Invention
The present invention relates to a demodulation circuit for signals modulated by the process of phase shift keying (hereinafter referred to as a PSK) or differential phase shift keying (hereinafter referred to as a DPSK).
2. Description of the Prior Art
The PSK and DPSK are modulation systems which belong to the class of continuous wave modulation systems (the continuous wave is hereinafter referred to as a CW) and in accordance with the PSK, during the demodulation the phase of the CW serving as a basis is compared with the phase of the modulated wave so as to discriminate, for example, whether the CW and the modulated signal are in phase or out of phase by 180.degree.. In accordance with the DPSK, if a bit is represented by several or several tens cycles of the CW, during the demodulation the phase of the modulated signal during the preceding bit is compared with the phase of the current modulated signal so as to discriminate whether the two signals are in phase or 180.degree. out of phase with each other.
In other words, with the known apparatus for the PSK or DPSK modulation, it has been important that the circuit for generating a reference signal for phase comparison operates stably and it has been difficult to make the necessary adjustments for the stable operation of the circuit.
Recently, in place of 35-mm film cameras, electronic still picture cameras have come to hold public attention in which a still picture signal corresponding to an image of an object is recorded on a recording medium such as a magnetic disc. With this type of electronic still picture camera, it is conceivable to record a still picture signal corresponding to an object image as well as photographic data such as the date of photographing on a recording medium. In the case of the cameras designed to FM modulate a still picture signal and record it on the medium, it has been proposed to DPSK modulate photographic information and record it on the medium. Thus, with an apparatus for reproducing the information on the medium, an essential matter is whether its DPSK demodulation circuit is readily adjustable, stable in operation and easy to manufacture as an IC.